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Question from simple lens diagram (camera)

  1. May 2, 2010 #1
    1. The problem statement, all variables and given/known data
    A simple camera has a converging lens of focal length 50.0 mm.

    It is used to photograph a tree 3.00 m high.

    The tree is 8.00 m away from the lens.

    Calculate the distance v between the lens and the film, giving your answer in mm to three significant figures.

    2. Relevant equations

    1/f = 1/u + 1/v

    3. The attempt at a solution

    1/50 = 1/8000 + 1/(-v)
    v = -50.3 mm

    Is this correct? Thank you! :smile:
  2. jcsd
  3. May 2, 2010 #2
    Why did you reason that the image distance was negative?
  4. May 2, 2010 #3
    Hmmm, not sure why I done that actually...

    Have corrected to:

    1/f = 1/u + 1/v

    1/50 = 1/8000 + 1/v

    v = 50.3 mm
  5. May 2, 2010 #4
    That looks right. The negative image distance you got before implies that the film needed to be outside the camera to capture the image.
  6. May 2, 2010 #5
    Thanks Squeezebox!

    When I want to calculate the linear magnification of the image, the following equation is used:

    m = v/u

    Do I use the same v and u values from above? (both in metres)


    0.0503 / 8 = 0.0063
  7. May 2, 2010 #6
    Now you have the opposite problem. There's a negative sign in this one.

    m= -v/u

    The image gets flipped upside down. This happens with your eyes too; your brain flips the image back right side up.
  8. May 2, 2010 #7
    Ah, so it'd be

    -0.0503 / 8 = -0.0063

    So is the height of the tree, stated in the question as being 3.00 m, not required to obtain the linear magnification?
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